This invention relates to combustion control apparatus for an internal combustion engine and particularly to such apparatus which includes means effective to control a combustion related engine parameter on the basis of the timing of peak combustion chamber pressure. Such systems in the past have generally used a differentiator and zero crossing detector circuit to determine the derivative of the combustion pressure wave form and generate a signal at the moment of peak pressure, as determined by the zero crossing of the derivative signal.
The detection of the true peak by this method is difficult, however, since the pressure signal wave form is generally contaminated with noise at a higher frequency which generates a multitude of peaks which hide the true peak. The prior art apparatus has therefore included filtering of various kinds in order to smooth the pressure wave form signal or its derivative sufficiently for detection of the timing of the actual peak pressure. However, in addition to the expense of these filters and the difficulty of accomplishing the filtering task in an effective manner, such filters generally introduce phase shifts to the pressure signal wave form which cause a shift in the apparent timing of the peak pressure.